An adequate supply of vitamin D is vital as the term “vitamin” already suggests. A deficiency of vitamin D leads to severe diseases such as rickets or osteoporosis. While vitamin D was still regarded as a single substance at the beginning of the last century, the vitamin D system has changed in the course of the last decades into a complex and manifold network of vitamin D metabolites. Nowadays more than 40 different vitamin D metabolic products are known (Zerwekh, J. E., Ann. Clin. Biochem. 41 (2004) 272-281).
Humans can only produce D3 vitamins or calciferols by the action of ultraviolet rays from sunlight on the skin. In the blood Vitamin D3 is bound to the so-called vitamin D-binding protein and transported to the liver where it is converted into 25-hydroxyvitamin D3 by 25-hydroxylation. A multitude of other tissues are nowadays known to be involved in vitamin D metabolism in addition to the skin and liver, the two organs that have already been mentioned (Schmidt-Gayk, H. et al. (eds.), “Calcium regulating hormones, vitamin D metabolites and cyclic AMP”, Springer Verlag, Heidelberg (1990) pp. 24-47). 25-Hydroxyvitamin D and more specifically 25-hydroxyvitamin D2 and 25-hydroxyvitamin D3 are the central storage form of vitamin D in the human organism with regard to their amounts. When needed these precursors can be converted in the kidneys to form the biologically active 1α,25-dihydroxyvitamin D the so-called D hormone. The biologically active vitamin D regulates among others calcium uptake from the intestine, bone mineralization and it influences a large number of other metabolic pathways such as e.g. the insulin system.
Measuring the vitamin D level itself is of little benefit when determining the vitamin D status of a patient, because concentrations of vitamin D (vitamin D2 and vitamin D3) fluctuate greatly depending on food uptake or exposure to sunlight. In addition vitamin D has a relatively short biological half-life in the circulation (24 hours) and it is therefore also for this reason not a suitable parameter for determining the vitamin D status of a patient. The same also applies to physiologically active forms of vitamin D (1,25-dihydroxyvitamin D). These biologically active forms also occur in relatively small and highly fluctuating concentrations (as compared to 25-hydroxyvitamin D which is disclosed and discussed in detail herein).